I want to build a loft above my shop for wood storage; vertical wood
posts, beams, joists and T&G plywood for the floor. The floor of the
shop is a slab on grade. I have plenty of height so there is no problem
I'm not sure what to do at the connection of the vertical posts and the
slab. Obviously there are no footings there to take posts. Can I just
butt the posts up to the slab and hope the weight doesn't screw up the
floor? What are other options? I'd like to avoid sawing the slab and
putting in footings unless I can't avoid it.
I'd at least spread the load across some thick steel plates - perhaps
18" square or so. Bolt the plate to the floor, then the post to the
plate with angle of some sort. I have no engineering expertise at all -
so my suggestions is purely a layman's "you get what you pay for"
Thought about tying the floor into the walls? Then add your posts as
"secondary support" With the bottoms of the posts attached just like you
would a porch/deck on concrete, you know those metal brackets.
Of course I would have to see to actually know what I am talking about so
disregard this if it feels unsafe.
"Violin playing and Woodworking are similar, it takes plenty of money,
I agree, it makes more sense to support the loft joists off the walls
and then reinforce the studs in the walls so that the load is carried
on the foundation, unless you want the loft to be free standing.
If one side is supported off the wall and the other on
posts then the posts (and the floor) are only supporting half the
weight. Supporting at least one side off the wall will also
help to prevent racking--which is really important. Building in
a corner or a mezanine accross an entire wall would be the most
stable way to go.
I've seen many sagging floor joists supported by pipe columns
with screw jacks on top (or sometimes on the bottom) supported
right on the basement slab and never saw the slab cracked around
I did about the same thing in my shop.
Shop is 28 x 40.......went to the rear wall area, marked out 3 post
points equal distance along the 28'. then 10' from the rear wall.
Drilled 3/4" holes 4" slab......determined height of post, cut posts
4x4's, drilled 3/4" holes in the end of posts, inserted steel pin 6
inches into post & 6 inches into floor holes.
Since i was building loft area by myself this step helped.
Put runners around sidewalls & end wall, levelling carefully, laid a
stringer joist across the 28' & 3 posts.........lap jointed the long
joist, 12" lap & epoxy glue.
Forgot to mention i notched the 4x4's to take the 2x8 stringer joist.
Lag bolted posts & stringer. put stirrup brackets 16" o/c on end wall &
stringer......did a real good fastening job !
lags not screws, drilled pilot holes and used epoxy.......overkill !
Set in 2x8 10' joists, end nailed on the stringer end as well as the
stirrup clinch points, toe nailed also on the end wall hangers.
Staggered the plywood joints for added strength, used a nail shooter.
So after 14 years and sometime 2 full pickup loads, bed rail high of
lumber up there, i have only about a 1/2 sag on the open end stringer
between posts. No cracks in concrete yet ?
4"s of slab will hold a lot of weight unless you're really banging it
run the numbers.
you need to know the section of the concrete: thickness, what steel
where, composition of the concrete.
you need to know how much weight is going to be sitting on it, and
the more you spread the weight out, the more you can get away with.
It has a lot to do with the codes in your area. If the shop isn't
attached to the house then it may not have to live up to code. What you
don't want is to do something, then sell the house. You will know how
much weight that you designed it to hold. The guy you sell the hous to
doesn't and he loads up the loft with a waterbed. The next thing is the
loft colapses and you get sued for not building to code.
You live out in rural Alabama, just go for it.
You might want to look at this site...
Depending on the weight you plan to have, concrete can take quite a load.
Seems the pieces you are missing is the expected load, and the expected
strength of what you have.
I'm guessing a bunch of wood isn't going to be a large load compared to the
what's necessary to support a whole building.
That said, and me knowing little to nothing about concrete, I would get
those two pieces and make some simple calculations. Off the top of my
head - a 4 inch slab might hold on the order of 2000 psi. Use a safety
factor of maybe 4 - just for starters, so 500 psi. Your 4x4 beam spans
about 12 in^2. So, that gives you about 6000 lbs of support per beam at the
concrete, but the 4x4 will buckle at that load... Anyway, the numbers are
usually just an indicator to be coupled with experience.
Seems to me, though, however many 4x4's you need to support your project,
the concrete will be stronger.
Might want to double check that with a quick phone call to a concrete guy.
Maybe get some more refined numbers or better design rules of thumb.
|I want to build a loft above my shop for wood storage; vertical wood
|posts, beams, joists and T&G plywood for the floor. The floor of the
|shop is a slab on grade. I have plenty of height so there is no problem
|I'm not sure what to do at the connection of the vertical posts and the
|slab. Obviously there are no footings there to take posts. Can I just
|butt the posts up to the slab and hope the weight doesn't screw up the
|floor? What are other options? I'd like to avoid sawing the slab and
|putting in footings unless I can't avoid it.
A lot of needed data is missing. Nevertheless, your concern seems to
be the load bearing capability of the concrete. If your slab was
built to code, the code requirement will tell you the specified
compressive strength requirement. Usually in the 3000 psi
neighborhood. This is a "28 day" requirement. Concrete continues to
gain strength with age.
I am not a structural engineer, so take what I say with a grain of
salt, but for a rough estimate, you can calculate the weight of the
structure and estimated loading and divide the total by the number of
load bearing columns and figure out the loading on the individual
columns. The psi is of course this number divided by the area of the
All said and done however, here's how I look at things like this. I
ask myself, "Self, if I want to punch a 4" x 4" hole in a slab of 4"
thick concrete using a wooded 4 x 4 post, how hard do I have to push?"
(Or 6x6, 8x8 or whatever)
And the answer is of course, that unless the substrate is mush and you
failed to marry the concrete with steel reinforcing (concrete without
steel is like a day without sunshine), the post will fail before the
concrete. Common structural Douglas Fir for instance has a
compressive strength of about 1200 psi parallel to the grain. (ref:
American Civil Engineering Practice, Vol III)
So your concern should be how many posts and of what size. And when
you get that figured out, you'll probably find that the horizontal
members are the weakest link ;)
It is good practice to put some steel between the columns and the
concrete so if you're still worried, make the plates bigger than the
columns and spread the load over a bigger area.
The thing you missing here is that the concrete may not be properly
supported. What is under it? Has moisture washed away some of the
gravel? Was the slap poored on fill and has had some settling? I would
not be worried about the load pushing a 4x4 through the concrete. I
would be woried about the weight in one place causing damage to the
slab. My harage has enough cracks in the slab floor. Adding another
few thousand pounds in one place would be a problem. I would rather cut
a hole and put in a footing. It is a pain but doable for a single
person. If you can for a hundred or so then why not? Especially since
the labor is free.
On Sun, 15 Feb 2004 00:35:03 GMT, Joe Willmann
|> wrote:|> |> All said and done however, here's how I look at things like this. I|> ask myself, "Self, if I want to punch a 4" x 4" hole in a slab of 4"|> thick concrete using a wooded 4 x 4 post, how hard do I have to push?"|> (Or 6x6, 8x8 or whatever)
|The thing you missing here is that the concrete may not be properly
|supported. What is under it? Has moisture washed away some of the
|gravel? Was the slap poored on fill and has had some settling? I would
|not be worried about the load pushing a 4x4 through the concrete. I
|would be woried about the weight in one place causing damage to the
|slab. My harage has enough cracks in the slab floor. Adding another
|few thousand pounds in one place would be a problem. I would rather cut
|a hole and put in a footing. It is a pain but doable for a single
|person. If you can for a hundred or so then why not? Especially since
|the labor is free.
Nothing missing at all. I also said, "And the answer is of course,
that unless the substrate is mush and you failed to marry the concrete
with steel reinforcing (concrete without steel is like a day without
sunshine), the post will fail before the concrete."
I built my garage on a lot of fill (>70 cu yd.) but it was engineered
fill, placed in 6" lifts, compacted to 2" and uses 1/2" rebar on 2'
centers. I have zero cracking, except in the grooves where it's
supposed to crack. Moving a 6,000 lb PU around hasn't seemed to hurt
Approaching this issue from this context, if I built a similar loft, I
couldn't afford enough lumber to damage the slab. And sawing through
my rebar to put in piers would be counter-productive.
But the OP wasn't talking about a garage, but a "shop" so we don't
know how the slab was engineered.
To do it properly, and it isn't really that hard, rent a concrete saw.
Cut a 12x12 hole through the concrete. You are probably talking a 4 - 8
hours work here. Now rent yourself a powered post hole digger. Go for
it. Then yse standard post hole diggers to clean out the hole. It is
good to make the hold wider at the bottom than the top.
Now poor concrete and add a couple of bolts to attach the verticle posts
Use engineered joists, no posts or beams needed. Manufacturer's will
tell you what spacing and depth of joists you need. This way the
outside walls will carry the load and you will have open area on first
floor.Are you removing roof first? If so the joists sit on wall plates
like usual.If roof stays then you you need a ribbon board ( ledger }
notched into the studs, or something that will carry the weight of the
joists.This will cut down headroom, although you say you have plenty
of height.I use the type of engineered joist that has Osb web and
laminated plywood nailing surface. Add a piece of 3/4" plywood to the
side that nails to the stud ( called crush block).This gives you solid
nailing thru joist into stud. Ribbon board can be a 8" rip of 3/4"
Another way is to use joist hangers if you have access to top plate,
and room to drop joist down into hangers.Your lumberyard can get you
in touch with engineered joist rep for more info.
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